The present invention relates generally to the field of boilers and steam generators and, in particular, to air heaters for heating combustion air.
The tubular air heater is the main air heating mechanism with the water coil air-heater (WCAH) as a commonly used alternative. A tubular air heater or WCAH is currently used to heat combustion air to a specified operating temperature. The full flow of the boiler's feedwater is used as the heat transfer medium when using the WCAH as the heat source. As the air is heated, the temperature of the feedwater is lowered. The feedwater leaving the WCAH is then sent to an economizer where it is used to lower the temperature of the flue gas of the boiler. In certain cases a tubular air-heater (TAH) in conjunction with a WCAH is used to obtain a lower final exit gas temperature. As the stack gas temperature decreases the size of the TAH and WCAH increases. The size of the air-heaters will increase substantially as the gas temperature drops below 325 degrees F. The current technology is limited by the feedwater temperature, the stack gas temperature, and the required combustion air temperature.
U.S. Pat. No. 3,818,872 to Clayton, Jr. et al. discloses an arrangement for protecting, at low loads, furnace walls of a once-through steam generator having a recirculation loop, by bypassing some of the incoming feedwater flow around the economizer of the arrangement.
U.S. Pat. No. 4,160,009 to Hamabe discloses a boiler apparatus containing a denitrator which utilizes a catalyst and which is disposed in an optimum reaction temperature region for a catalyst of the denitrator. In order to control the temperature of the combustion gas in the optimum reaction temperature region, this region is adapted to communicate with a high temperature gas source or a low temperature gas source through a control valve.
U.S. Pat. No. 5,555,849 to Wiechard et al. discloses a gas temperature control system for the catalytic reduction of nitrogen oxide emissions where, in order to maintain a flue gas temperature up to the temperature required for the NOx catalytic reactor during low load operations, some feedwater flow bypasses the economizer of the system by supplying this partial flow to a bypass line to maintain a desired flue gas temperature to the catalytic reactor.
Published Patent Applications US 2007/0261646 and US 2007/0261647 to Albrecht et al., the disclosures of which are hereby incorporated by reference as though fully set forth herein, disclose a multiple pass economizer and method for SCR temperature control where maintaining a desired economizer outlet gas temperature across a range of boiler loads comprises a plurality of tubular configurations having surfaces that are in contact with the flue gas. Each tubular configuration may comprise a plurality of serpentine or stringer tubes arranged horizontally or vertically back and forth within the economizer, and each tubular configuration has a separate feedwater inlet.
Current technologies typically supply flue gas at or near the stack of the boiler system at well above 300 degrees F. It would be advantageous if a system were discovered that could economically lower this flue gas exit temperature.